Bahamas islands were giant labs for lizard experiment

Wrapping entire islands in the Bahamas with netting, introducing snakes to two other islands and measuring the fitness of hundreds of lizards using treadmills&colon; one of the most ambitious ecological field experiments ever conducted has resolved a long-standing question about the evolution of lizards.

Lizards of the genus Anolis are found throughout the American tropics, where they vary widely in size and shape depending on ecological conditions. It has long been thought that predation is the most important evolutionary force for continental lizard populations, whereas on islands competition between lizards themselves is more important. Until now, though, no one had tested this experimentally.

Ryan Calsbeek and Robert Cox of Dartmouth College in Hanover, New Hampshire, cut no corners in their experiment. They excluded predators from two small, uninhabited islands in the Bahamas by wrapping the islands – about 1000 square metres each – with netting to keep out predatory birds. Meanwhile, they enhanced predation on two other islands by introducing lizard-eating snakes.

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To vary the amount of competition, they seeded one of each pair of islands with high densities of Anolis sagrei lizards, and the other with lower densities of the animals.

Lance Armstrong lizards

But first, they got to know the lizards. Before release, they marked and measured each one and tested its stamina by running it to exhaustion on a treadmill.

“Your Lance Armstrong lizards can run about 7 minutes. Your overweight field-biologist lizard runs for about 2 minutes,” says Calsbeek. “We spent several hours a day just running the animals, and we did that day in and day out for several weeks.”

Four months later, the researchers returned to the island and recaptured every remaining lizard, noting which had survived and which died. Larger, longer-legged and higher-stamina lizards had survived better than smaller, wimpier ones on higher-density islands where competition was more intense, they found. However, these traits did not affect the chance of survival in the face of predation. This supports the idea that competition, and not predation, is the primary selective force in these island lizards, says Calsbeek.

“To me, that’s surprising,” says David Reznick, an evolutionary biologist at the University of California, Riverside. “I would have thought that predation would matter.”